1. Chapter 8 Heat Transfer & Change of Phase

2. Heat Transfer
How does the energy move from a hotter to a colder object?
Three mechanisms
Conduction
Convection
Radiation

3. Conduction Stir your hot soup with a metal spoon
Pretty soon you need a pot holder because the end of the spoon you are holding gets hot
This is heat transfer by conduction
Energy travels up the spoon from the end in the hot soup to the end in your hand

4. Conduction
We sense the movement of energy by the increasing temperature
This means the atoms and molecules have higher average kinetic energy
Primarily occurs by the movement of electrons in the material
The more easily the electrons can move, the better the conduction

5. Conduction
Metals have some electrons that are very loosely bound to the atoms in the material
These electrons can move easily and can rapidly pick up additional kinetic energy
Metals are good conductors
Wood and plastic don’t have loosely bound electrons, so they are poor conductors

6. Conduction

7. Conduction Air is a poor thermal conductor
If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warm
Snow is a poor conductor, while water is better
Makes igloos a useful as a house

8. Convection A phenomenon in fluids
Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a current
These moving fluid currents are convection

9. Convection

10. Convection
When the radiator heats the air, it becomes less dense and rises
Cool air moves in to replace the air that rose
This generates the air flow
So radiators don’t need a fan to stir the air and to distribute heat throughout a room
The rising air cools until its density matches that of the surrounding air

11. Convection
We take advantage of the cooling that occurs during an expansion
We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space

12. Convection
Explains why breezes come from the ocean in the day and from the land at night

13. Radiation Energy carried by electromagnetic waves
Light, microwaves, radio waves, x-rays
Wavelength is related to vibration frequency

14. Radiation

15. Radiation
Every object is emitting electromagnetic waves regardless of temperature
Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum
Most things emit primarily in the infrared
Night vision goggles, etc.

16. Radiation Things also absorb radiation
If they didn’t, they would run out of energy to emit
Good emitters are also good absorbers
Equilibrium established between emission and absorption
When something can’t equilibrate, it gets hotter or colder

17. Radiation Interior of a car on a sunny day
Sunlight comes in as visible light
Seats and interior are much cooler so they radiate in the infrared instead of visible
Glass in the windows blocks infrared so energy can’t get out
Car interior heats up!

18. Radiation A good absorber reflects very little energy
Think about dark pavement
A poor absorber reflects a lot of energy
Think about snow that doesn’t melt in sunshine even though 1400 watts/meter2 are hitting it

19. Radiation At night, objects receive no input energy from the sun
But, they are warmer than outer space, so they continue to radiate energy
Thus, they cool off
Can we make ice in the desert without a refrigerator?

20. Newton’s Law of Cooling
Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings
Works both ways, cooling and heating
Rate of heating also depends on the temperature difference

21. Phases of Matter
Four States
Solid
Liquid
Gas
Plasma

22. Solids
Atoms or molecules are well ordered into a crystalline lattice for most materials
Exceptions are things like glass which is really a very slow flowing liquid
Metallic Solids don’t need all their electrons for bonding
Insulators need all their electrons for bonding

23. Liquids Pretty incompressible
Atoms or molecules loosely bonded to neighbors, but in close proximity
Allows fluid flow

24. Gases
Compressible
Atoms or molecules are widely separated and have little interaction with each other aside from elastic collisions

25. Plasmas
Temperature is so high that outer electrons are separated from their underlying atoms
No molecules exist
Particles are electrically charged
Sun, stars, inside a fluorescent bulb

26. Changing States For a given substance, all four states are possible
Increase temperature and a PHASE CHANGE occurs
Solid > Liquid > Gas > Plasma

27. Phase Changes
Evaporation
Condensation
Boiling
Melting
Freezing

28. Water Molecule
Two Hydrogen Atoms, One Oxygen Atom - - + +

29. Evaporation

30. Evaporation
Liquid cools because it loses higher energy molecules or atoms
Air cools because the molecules leaving give up a lot of energy to break free, so they have much lower energy after they gain their freedom
Solid > Gas directly is sublimation

31. Condensation

32. Condensation The opposite of evaporation
Evaporation and Condensation are a SURFACE phenomenon
Interaction of molecules at the liquid-air interface

33. Boiling
Molecules break free of the liquid bonds and form little bubbles
If the vapor pressure in the bubble is large enough to resist the pressure of the liquid, and the air pressure above the interface they rise and escape to the outside
Remember, liquid is cooling and air is cooling

34. Boiling

35. Boiling

36. Melting and Freezing Change between liquid and solid
Bonds between molecules break when solid melts
Takes energy to break the bonds
Freezing is the opposite of melting
Molecules move slowly enough that bonds can form

37. Melting and Freezing
Why does salt melt ice?

38. Energy and Phase Change
Adding heat at a constant rate T
Time

39. Energy and Phase Change
Adding heat at a constant rate T
Boiling
Melting
Time

40. Energy and Phase Change

41. Energy and Phase Change
For Pure Water
334 joules/gram
2256 joules/gram

42. Boiling Potatoes
So, when you want garlic mashed potatoes and you take potatoes and cut them up and put them in boiling water, do they cook faster when the water is really bubbling compared to when the water is just barely bubbling???

43. Boiling Potatoes